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ARP Caching

ARP Caching. Christopher Avilla. What is ARP all about?. Background Packet Structure Probe Announcement Inverse and Reverse Proxy Tools Poisoning MAC Flooding. ARP Refresher. Determines a MAC when only IP address is known I mplemented in many types of networks

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ARP Caching

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  1. ARP Caching Christopher Avilla

  2. What is ARP all about? • Background • Packet Structure • Probe • Announcement • Inverse and Reverse • Proxy • Tools • Poisoning • MAC Flooding

  3. ARP Refresher • Determines a MAC when only IP address is known • Implemented in many types of networks • Most frequently used to translate IPv4 addresses into Ethernet MAC addresses • In the next generation Internet Protocol, IPv6, ARP's functionality is provided by the Neighbor Discovery Protocol (NDP).

  4. Packet Structure • Simple message format • One address resolution request or response • Operation code for request (1) and reply (2) • 4 addresses, the hardware and protocol address of the sender and receiver

  5. Probe • ARP request constructed with an all-zero sender IP address • IPv4 Address Conflict Detection specification (RFC 5227). • First test to see if the address is already in use, by broadcasting ARP probe packets.

  6. Announcements • gratuitous ARP message • Updating other host's mapping of a hardware address when the sender's IP address or MAC address has changed • broadcast as an ARP request containing the sender's protocol address (SPA) in the target field (TPA=SPA), with the target hardware address (THA) set to zero. • An alternative is to broadcast an ARP reply with the sender's hardware and protocol addresses (SHA and SPA) duplicated in the target fields (TPA=SPA, THA=SHA).

  7. Announcements Cont. • Not intended to solicit a reply • Updates any cached entries in the ARP tables of other hosts that receive the packet. • Many operating systems perform gratuitous ARP during startup • Load balancing for incoming traffic • In a team of network cards, used to announce a different MAC address within the team that should receive incoming packets.

  8. Inverse ARP • Protocol used for obtaining IP addresses from MAC addresses • Used in Frame Relay and ATM networks • As ARP translates Layer 3 addresses to Layer 2 addresses, InARP may be described as its inverse • Implemented as a protocol extension to ARP • Uses the same packet format from ARP with different operation codes.

  9. Reverse ARP • Translates Layer MAC addresses to IP addresses • Used to obtain the IP address of the requesting station itself for address configuration purposes • RARP is now obsolete. It was replaced by BOOTP, which was later superseded by the Dynamic Host Configuration Protocol (DHCP).

  10. Proxy • Device on a given network answers the ARP queries for an IP address not on that network • The ARP Proxy is aware of the location of the traffic's destination • Offers its own MAC address in reply • "send it to me, and I'll get it to where it needs to go." • The "captured" traffic is routed by the Proxy to the intended destination via another interface or tunnel • Sometimes referred to as 'publishing'.

  11. Tools • ARPwatch • Generates a log of observed pairing of IP addresses with MAC addresses along with a timestamp when the pairing appeared on the network. • ARPing • The program tests whether a given IP address is in use on the local network, and can get additional information about the device using that address • Cain and Able

  12. Cache Poisoning • Update cache whenever an ARP request OR!!! Reply is received. • If the MAC address for the given IP has changed. Overwrite the old value • ARP replies are unicast • Used to set up man in the middle attacks • Allows attacker to monitor, intercept, and modify sessions

  13. MAC Flooding • ARP Cache Poisoning technique • For Network switches • When certain switches are overloaded they often drop into a "hub" mode. • The switch is too busy to enforce its port security features and just broadcasts all network traffic • Flood a switch's ARP table with a ton of spoofed ARP replies then packet sniff

  14. Why do we care? • Network Design • Security • Device Configuration • Advanced Devices • Nortel • Cisco • Allied Tellesis

  15. Conclusion • Packet Structure • Probes and Announcements • Extensions of the protocol • Tools • Threats

  16. Resources • http://www.packetnexus.com/docs/arppoison.pdf • http://en.wikipedia.org/wiki/Address_Resolution_Protocol • http://www.watchguard.com/infocenter/editorial/135324.asp

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